1. Field of the Invention
This invention relates generally to a belted-alternator-starter (BAS) system for a vehicle and, more particularly, to a BAS system for a vehicle, where the BAS system includes a motor device that receives a minimal field current when the vehicle ignition is on and the engine is off so as to quickly provide engine torque when the engine is restarted.
2. Discussion of the Related Art
Vehicles employ alternators that are driven by a belt coupled to the vehicle engine to provide electrical power. The alternator includes a rectifier to convert AC current to DC current to charge the vehicle battery. The field current of the alternator is regulated to provide the proper battery charge. Particularly, claw-pole, wound rotor AC synchronous devices are used in combination with a rectifier and a field current regulator in vehicles as a belt-driven generator. Permanent magnets have been employed in the claw-pole device to increase the power output and efficiency of the alternator for a given alternator size.
Certain state of the art vehicle designs have investigated using the alternator as a starter motor to start the engine so that the vehicle engine can be turned off when the vehicle is stopped, such as at a stop light, to conserve fuel. These devices are typically known as belted-alternator-starters (BAS). The torque required to start the engine when it is warm is much less than the torque required when the engine is cold. Therefore, a starting device is required to provide the necessary high torque for cold starts. In conventional power-trains, the starter motor provides this torque and starts the engine relatively slowly. Because the alternator is directly connected to the engine by the belt, and thus has a smaller pulley ratio compared to the gear ratio of the starter motor, it has to be designed to produce higher torque not only to take care of cold starts, but also to accelerate the engine quickly so that the starting is transparent to the driver.
In the known vehicles employing a BAS, the three-phases of the AC synchronous alternator are connected to a three-phase active bridge circuit, which functions as a controlled rectifier when the BAS is in the generator mode and as an inverter when the BAS is in the motor mode.
Magnetic flux generated by the permanent magnets and/or the field coil in the rotor induces a current in the armature coils. The interaction of the flux and current gives the alternator its torque. The known BAS system provided the field current and the armature current simultaneously from the power-train controller commands at the start of the vehicle engine. However, the torque response of the engine under this control is unacceptably slow, on the order of 400–600 milliseconds. It would be desirable to provide a BAS that generated the torque more quickly at engine start-up so that the re-starting of the engine at, for example, a stop-light is transparent to the driver of the vehicle.